Developments in combustion engine technology have shown that compression ignition engines, frequently referred to as diesel-cycle engines, can be fuelled by gaseous fuels instead of diesel without sacrifices in performance or efficiency. Examples of such fuels include natural gas, methane, propane, ethane, gaseous combustible hydrocarbon derivatives and hydrogen. Substituting diesel with such gaseous fuels generally results in cost, availability and emissions benefits over diesel.
However, one challenge in using gaseous fuels for such applications is that the energy density of gaseous fuels is not as high as conventional liquid fuels. This is an important consideration, for example, with gaseous fuel systems employed for vehicular applications, because fuel storage space on a vehicle is limited. For gaseous-fuelled vehicles to be attractive versus conventional diesel-fuelled vehicles, the on-board fuel storage system should not diminish the vehicle's transport capacity.
To increase the energy density of a gaseous fuel, it can be stored at high pressure. To contain a gas at high pressure, a pressure vessel rated for a specified maximum holding pressure must be used. For gaseous fuels, compared to a compressed gas, higher energy densities can be achieved at lower storage pressures with a liquefied gas. As a result, the fuel tank does not need to be rated for as high a pressure, which can reduce the weight of the fuel tank.
Accordingly, a preferred method of increasing the energy density of a gaseous fuel is to store it in a liquefied state at cryogenic temperatures. A liquefied gas stored at a cryogenic temperature is referred to herein as a cryogenic fluid and a gaseous fuel stored in a liquefied state at a cryogenic temperature is referred to herein generally as a “cryogenic fuel”.
For the purposes of this application, cryogenic fuels include liquefied gaseous fuels that boil at temperatures at or below −100° C. under atmospheric pressures. An example of such fuel is liquefied natural gas, commonly known as “LNG”.
In the present disclosure LNG is referred to as a preferred example of a cryogenic fuel because of the vast proven reserves of natural gas in many of the potential markets around the world, the affordability of natural gas, and the already existing infrastructure for natural gas, which is continuing to expand in breadth and capacity.
However, people skilled in the technology would understand that the present storage container can be employed to hold other cold or cryogenic fuels or liquefied gases generally. By way of example, the disclosed storage container could be employed to accommodate other hydrocarbons such as methane, ethane, propane and hydrocarbon derivatives or non-organic fuels such as hydrogen. Furthermore, the container that is the subject of this disclosure can also be used to hold other liquefied gases at cryogenic temperatures, such as helium, nitrogen and oxygen.
However, one of the challenges of storing liquefied gas at cryogenic temperatures is reducing heat transfer into the cryogen space. As the temperature of the liquid increases, the vapour pressure rises inside the storage container. Cryogenic storage containers are normally equipped with a pressure relief venting system to prevent over-pressurization of the storage container. Excessive heat transfer into a cryogen space can result in fuel wastage through venting. When the liquefied gas is a fuel, it is also undesirable to routinely release fuel into the environment. With natural gas used as an example, methane, a major component of natural gas, is a greenhouse gas.
Cryogenic storage containers commonly use a double walled construction with an inner vessel, which holds the liquefied gas, suspended inside an outer vessel. A vacuum applied to the space between the inner vessel and the outer vessel minimizes conductive and convective heat transfer. It is also known to wrap sheets of a super-insulating material around the inner vessel to minimize radiant heat transfer. However, the structural supports for the inner vessel and any piping between the inner vessel and outside environment, all provide heat conduction paths and the transfer of heat energy to the liquefied gas in the cryogen space from the outside environment is commonly known as “heat leak”.
As long as there are structural supports for the inner vessel and there are pipes or conduits that penetrate through the insulated space, some heat leak will occur. “Holding time” is defined herein as the time span that a cryogen can be held inside the storage container before heat leak into the cryogen space causes the vapour pressure to rise to a level at which the pressure relief valve opens. Accordingly, holding times can be extended without the need to vent excess vapor pressure if heat leak can be reduced.
U.S. Pat. No. 5,651,473 discloses a support system for cryogenic vessels that is representative of the current state of the art. As shown in FIG. 1 and the enlarged view of FIG. 1A, an arrangement is provided for supporting inner vessel 102 within outer vessel 104, while also providing an opening through which conduits can be inserted so that fluid can flow into and out of the cryogen space.
Prior art assembly 110 consists of manifold block 112 that supports inner cylindrical member 114. Collar 118 is fixed to the opposite end of member 114 to define interior space 116 (see FIG. 1A). A passageway can be provided in block 112 or cylindrical member 114 to communicate space 116 with insulation chamber 106 so that when a vacuum is created in insulation chamber 106 it will also be created in space 116. Collar 118 supports a second cylindrical member 123 that is disposed over and is coaxially aligned with member 114. The space 125 between cylindrical members 114 and 123 also communicates with insulation chamber 106. A plurality of pipes 122, 126, and 130 extend between collar 118 and manifold block 112. These pipes are provided with a bend, sometimes referred to as a “joggle”, which provides a trap to create a liquid/vapor interface and allows for differential thermal expansion or contraction between the pipes and cylindrical member 114. Pipe 122 is connected to liquid fill line 124, pipe 126 is connected to liquid delivery line 128, and pipe 130 is connected to vent 132.
A disadvantage of this system is that cylindrical member 114 and the pipes are both metallic thermal conductors and both penetrate the insulated space, providing a plurality of heat paths through which heat can be introduced into the cryogen space. Cylindrical member 114 is metallic so that it can be welded and sealed to manifold block 112 and collar 118, but less heat transfer could be achieved if a structural material with a lower thermal conductivity could be employed.
In addition, cylindrical member 114 provides support in both the radial and axial directions, so the wall thickness of cylindrical member 114 must be designed to provide adequate strength in all directions, which is an important consideration in mobile applications because the momentum of the inner vessel and the cryogenic fluid contained therein is affected by the vehicle's acceleration, deceleration and changes in direction. Consequently, a further disadvantage of the prior art arrangement of FIGS. 1 and 1A is that a thicker wall for cylindrical member 114 correlates to higher thermal conductivity because the cross-sectional area through which heat transfer can occur is greater.